Most animals you’re familiar with probably stop growing after a few years, around the time when they reach sexual maturity.
Most mammals (maybe not all!) have genetic programming that puts a size limit on how far they’ll grow when they reach maturity. As usual, scientists have a name for it: it’s called determinate growth.
Some sharks appear to keep growing throughout their lives, exhibiting indeterminate growth. This is thought to be beneficial to heat retention and predator avoidance, though recent evidence is calling into question whether sharks do continue growing their entire lives.
Determinate vs. Indeterminate growth
Determinate-growth animals and plants have pre-set biological limits to their size, no matter how long they live. There’s an advantage to this – if you never stop growing, and you live a long time, you’re going to need more and more food until you can’t find anything to eat! So, evolution has selected animals that don’t get bigger than they need to be in order to survive.
Animals who stop growing after sexual maturity typically have growth plates in their long bones, like the arms and legs that fuse together over time and lock in place.
But there are some animals for whom this isn’t the case. Until recently, it was thought that Squamates (snakes and lizards) were all capable of life-long or indeterminate growth. While this idea has now been rejected, there are still some members of the group who seem to just… keep… growing…
In cold-blooded animals, there is a tendency to have slower, longer, and possibly indeterminate growth, and there are still some reptile species that seem to continue growing their entire lives. But it’s a critical time for the concept, as more and more animals are carefully studied, opinions around indeterminate growth are changing. At least in reptiles.
Some sharks are thought to exhibit indeterminate growth.
As long-lived iteroparous (reproducing multiple times) animals, they are prime candidates for either of the two growth strategies, but with the lack of data on sharks in general, it’s up for debate as to whether their growth is truly indeterminate.
Why do sharks never stop growing?
In the 1960s, Niko Tinbergen, a Dutch biologist, pointed out that there are at least four ways to answer a ‘why’ question.
He devised a system of breaking curiosity into four different approaches. It’s mostly designed for behavior studies, but it works well with almost any ‘why’ in animal biology too.
Essentially, this particular query can be broken into the following questions:
- Function – How does the endless growth benefit the shark?
- Evolution – How did endless growth evolve in sharks?
- Causation – What is the mechanism behind this growth?
- Development – In what way has this attribute been affected by the life of the shark?
[This is a very brief summary of the concept – there isn’t enough time to cover all of them in too much detail, but it’s a fascinating and useful practice, so if you’re interested, read more here.]
So, for this single ‘why’ question, you get four different answers depending on how you approach it. Let’s go through them.
- Why do sharks never stop growing? Function answer:
Indeterminate growth can be hugely beneficial for cold-blooded or ectothermic animals like sharks because a larger body size equates to greater thermal inertia. The larger an animal is, the less body heat it loses through its skin.
And it’s not just ectotherms. Water conducts body heat away from animals a lot faster than air does so that it can be equally beneficial to endotherms too!
Another benefit to being huge is that nobody messes with you.
- Why do sharks never stop growing? Evolution answer:
Considering that indeterminate growth may have been the norm in prehistoric animals, this might be better phrased as ‘why do other animals than sharks stop growing?’. It seems like back in the day, everyone was growing as big as possible and now only a handful of ancient lineages hold onto it as a beneficial strategy.
So, if this evolved first, then sharks are the OGs of indeterminate growth, and everyone else has forgotten about it.
- Why do sharks never stop growing? Causation Answer:
In hard-boned, determinate growers, the fusion of growth plates puts a stop to their size around sexual maturity.
In indeterminate growers, these plates never fuse, and so the animals just get bigger and bigger (though growth does appear to slow down).
Sharks have cartilaginous skeletons, and their growth is quite complicated, but they don’t have growth plates at all. Instead, tessellated sheaths surround the cartilage, and there doesn’t seem to be a lot of data on a specific mechanism for fusion in indeterminate growth; determinate growth may instead be an inhibited form of this ancestral norm.
- Why do sharks never stop growing? Development Answer
The size of the environment in the ocean and the lack of much gravitational effect means that animals can get a lot larger in the ocean than they do on land. For example, if a blue whale were placed on the earth, it would be crushed by its own weight and die. These mechanical restrictions aren’t such an issue in the water.
They also have plenty of space in their environment. Since there are no spatial restrictions, and they live for a long time and have plenty of food, why not get swole?
Do sharks really never stop growing?
As mentioned, this whole determinate vs. indeterminate growth thing is still a bit unclear to scientists. People (especially biology teachers) like to put distinctions between two things that are often not a dichotomy (see the cold-blooded/Warm-blooded discussion here).
While this is good for simplifying information and quickly getting through a syllabus, it can lead to muddy definitions and more exceptions than rules when nobody can agree on a strict example of either extreme.
Growth seems to be on a bit of a spectrum like this, rather than one way or another.
Many animals that were previously considered indeterminate growers are turning out not to be, and many that still are, seem to slow in growth considerably to the point where you wonder if they’re really getting any bigger at all.
Whale sharks were thought to grow bigger and bigger forever until people started making better records of their photos and realized that most of the measurements had been taken on juveniles. Now there’s quite a bit of evidence suggesting they don’t.
It’s not just fish either – orangutans were once thought to exhibit endless growth when in actuality, they were just getting fat.
However, with the fantastic diversity in sharks, it seems likely that there’s something for everyone, and if you look hard enough, there’s a chance you’ll find at least one shark that never stops growing.
Indeterminate growth has been a widely-accepted phenomenon in some cold-blooded animal groups for a long time. However, its universality has recently come into question as measurement and recording methods become more accurate.
Some sharks are still thought to exhibit indeterminate growth, and the current evidence does suggest that this is the case.
Still, simply by remembering how difficult it is to know anything about these elusive and mysterious animals, it’s easy to keep an open mind to the validity of these claims.